In addition, we added support for a programmable segment map, which enables psychovisual quality optimizations and defining region-of-interests. This means we can for example code the foreground objects (i.e. people) with a better quality (smaller quantizer) than the static background. We also added new hooks to the hardware that allows us to improve the quality of the encoder by later firmware upgrades that optmize our cost function algorithms - even after the chip has been manufactured.
In terms of silicon usage, Blueberry costs 13% more logic gates than Anthill, while the internal memory requirement remains unchanged. We optimized the maximum attainable clock frequency from Anthill’s 376 MHz to 392 MHz (TSMC 65nm, LP), which allows the chip manufacturer to get some more fps, which can be useful if you are doing multiple simultaneous encodes or running in a slow-motion mode (i.e. VGA 200 fps).

Comparing the quality difference between Anthill and Blueberry, we measured their average PSNR and SSIM quality over 46 test sequences and at a wide quantizer range. A few example results are shown below (positive numbers mean Blueberry was better):

Sequence

Resolution

PSNR [dB]

SSIM

city

qcif

+0.80

+0.033

table

qcif

+0.86

+0.009

ice

qcif

+0.89

+0.005

suzie

qcif

+0.82

+0.013

crew

cif

+0.46

+0.012

ice

cif

+1.21

+0.006

crew

4cif

+0.48

+0.010

soccer

4cif

+0.70

+0.022

video_conferencing

720p

+1.14

+0.006

rush_hour

1080p

+0.92

+0.004

pedestrian_area

1080p

+1.09

+0.013

whale_show

1080p

+0.21

+0.006

sunflower

1080p

+1.68

+0.007

As our focus in the improvement work has been on the video conferencing use case, let’s dig a bit deeper there. The following graph shows PSNR quality metrics for a 720p video call, comparing the H1 Blueberry release to Anthill and libvpx Bali release in different complexity modes (higher is better).

From the graph it can be seen that the Blueberry release encodes the video conference content at the same quality using up to 30% less bits than Anthill. It also beats libvpx’s simplest real-time setting at a much lower bitrate than before.

While more improvements are on the way for the third release of the H1 encoder, the current performance is already very competitive - and the hardware now comes with hooks for further software-based optimizations.

Aki Kuusela is Engineering Manager of the WebM Project hardware team in Oulu, Finland.